The present invention relates to optimizing DNA sequences in order to (a) improve the properties of a protein of interest by artificial generation of genetic diversity of a gene encoding the protein of interest by the use of the so-called gene- or DNA shuffling technique to create a large library of xe2x80x9cgenesxe2x80x9d, expressing said library of genes in a suitable expression system and screening the expressed proteins in respect of specific characteristics to determine such proteins exhibiting desired properties or (b) improve the properties of regulatory elements such as promoters or terminators by generation of a library of these elements, transforming suitable hosts therewith in operable conjunction with a structural gene, expressing said structural gene and screening for desirable properties in the regulatory element.
It is generally found that a protein performing a certain bioactivity exhibits a certain variation between genera and even between members of the same species differences may exist. This variation is of course even more outspoken at the genomic level.
This natural genetic diversity among genes coding for proteins having basically the same bioactivity has been generated in Nature over billions of years and reflects a natural optimization of the proteins coded for in respect of the environment of the organism in question.
In today""s society the conditions of life are vastly removed from the natural environment and it has been found that the naturally occurring bioactive molecules are not optimized for the various uses to which they are put by mankind, especially when they are used for industrial purposes.
It has therefore been of interest to industry to identify such bioactive proteins that exhibit optimal properties in respect of the use to which it is intended.
This has for many years been done by screening of natural sources, or by use of mutagenesis. For instance, within the technical field of enzymes for use in e.g. detergents, the washing and/or dishwashing performance of e.g. naturally occurring proteases, lipases, amylases and cellulases have been improved significantly, by in vitro modifications of the enzymes.
In most cases these improvements have been obtained by site-directed mutagenesis resulting in substitution, deletion or insertion of specific amino acid residues which have been chosen either on the basis of their type or on the basis of their location in the secondary or tertiary structure of the mature enzyme (see for instance U.S. Pat. No. 4,518,584).
In this manner the preparation of novel polypeptide variants and mutants, such as novel modified enzymes with altered characteristics, e.g. specific activity, substrate specificity, thermal, pH and salt stability, pH-optimum, pI, Km, Vmax etc., has successfully been performed to obtain polypeptides with improved properties.
For instance, within the technical field of enzymes the washing and/or dishwashing performance of e.g. proteases, lipases, amylases and cellulases have been improved significantly.
An alternative general approach for modifying proteins and enzymes has been based on random mutagenesis, for instance, as disclosed in U.S. Pat. No. 4,894,331 and WO 93/01285.
As it is a cumbersome and time consuming process to obtain polypeptide variants or mutants with improved functional properties a few alternative methods for rapid preparation of modified polypeptides have been suggested.
Weber et al., (1983), Nucleic Acids Research, vol. 11, 5661, describes a method for modifying genes by in vivo recombination between two homologous genes. A linear DNA sequence comprising a plasmid vector flanked by a DNA sequence encoding alpha-1 human interferon in the 5xe2x80x2-end and a DNA sequence encoding alpha-2 human interferon in the 3xe2x80x2-end is constructed and transfected into a rec A positive strain of E. coli. Recombinants were identified and isolated using a resistance marker.
Pompon et al., (1989), Gene 83, p. 15-24, describes a method for shuffling gene domains of mammalian cytochrome P-450 by in vivo recombination of partially homologous sequences in Saccharomyces cerevisiae by transforming Saccharomyces cerevisiae with a linearized plasmid with filled-in ends, and a DNA fragment being partially homologous to the ends of said plasmid.
In WO 97/07205 a method is described whereby polypeptide variants are prepared by shuffling different nucleotide sequences of homologous DNA sequences by in vivo recombination using plasmidic DNA as template.
U.S. Pat. No. 5,093,257 (Assignee: Genencor Int. Inc.) discloses a method for producing hybrid polypeptides by in vivo recombination. Hybrid DNA sequences are produced by forming a circular vector comprising a replication sequence, a first DNA sequence encoding the amino-terminal portion of the hybrid polypeptide, a second DNA sequence encoding the carboxy-terminal portion of said hybrid polypeptide. The circular vector is transformed into a rec positive microorganism in which the circular vector is amplified. This results in recombination of said circular vector mediated by the naturally occurring recombination mechanism of the rec positive microorganism, which include prokaryotes such as Bacillus and E. coli, and eukaryotes such as Saccharomyces cerevisiae. 
One method for the shuffling of homologous DNA sequences has been described by Stemmer (Stemmer, (1994), Proc. Natl. Acad. Sci. USA, Vol. 91, 10747-10751; Stemmer, (1994), Nature, vol. 370, 389-391). The method concerns shuffling homologous DNA sequences by using in vitro PCR techniques. Positive recombinant genes containing shuffled DNA sequences are selected from a DNA library based on the improved function of the expressed proteins.
The above method is also described in WO 95/22625. WO 95/22625 relates to a method for shuffling of homologous DNA sequences. An important step in the method described in WO 95/22625 is to cleave the homologous template double-stranded polynucleotide into random fragments of a desired size followed by homologously reassembling of the fragments into full-length genes.
A disadvantage inherent to the method of WO 95/22625 is, however, that the diversity generated through that method is limited due to the use of homologous gene sequences (as defined in WO 95/22625).
Another disadvantage in the method of WO 95/22625 lies in the production of the random fragments by the cleavage of the template double-stranded polynucleotide.
A further reference of interest is WO 95/17413 describing a method of gene or DNA shuffling by recombination of specific DNA sequencesxe2x80x94so-called design elements (DE)xe2x80x94either by recombination of synthesized double-stranded fragments or recombination of PCR generated sequences to produce so-called functional elements (FE) comprising at least two of the design elements. According to the method described in WO 95/17413 the recombination has to be performed among design elements that have DNA sequences with sufficient sequence homology to enable hybridization of the different sequences to be recombined.
WO 95/17413 therefore also entails the disadvantage that the diversity generated is relatively limited. Furthermore the method described is time consuming, expensive, and not suited for automatisation.
Despite the existence of the above methods there is still a need for better iterative in vitro recombination methods for preparing novel polypeptide variants. Such methods should also be capable of being performed in small volumes, and amenable to automatisation.
The present invention concerns briefly the utilization of template shift of a newly synthesized DNA strand during in vitro DNA synthesis in order to achieve DNA shuffling. By using this technique it is possible to obtain such results in a more expedient manner, and to some extent even a greater variation than in the above mentioned methods.
The method of the invention is also very well suited for adaption to automatisation.
In a preferred embodiment the technique is used in combination with an error-prone polymerase thereby introducing an even greater variation in the library created.
More specifically the present invention relates to a method for the construction of a library of recombined homologous polynucleotides from a number of different starting single or double stranded parental DNA templates and primers by induced template shifts during an in vitro polynucleotide synthesis using a polymerase, whereby
A. extended primers or polynucleotides are synthesized by
a) denaturing parental double stranded DNA templates to produce single stranded templates,
b) annealing said primers to the single stranded DNA templates,
c) extending said primers by initiating synthesis by use of said polymerase,
d) cause arrest of the synthesis, and
e) denaturing the double strand to separate the extended primers from the templates,
B. a template shift is induced by
a) isolating the newly synthesized single stranded extended primers from the templates and repeating steps A.b) to A.e) using said extended primers produced in (A) as both primers and templates, or
b) repeating steps A.b) to A.e),
C. the above process is terminated after an appropriate number of cycles of process steps A. and B.a), A. and B.b), or combinations thereof, and
D. optionally the produced polynucleotides are amplified in a standard PCR reaction with specific primers to selectively amplify homologous polynucleotides of interest.
In specific embodiments various modifications can be made in the process of the invention. For example it is advantageous to apply a defective polymerase either an error-prone polymerase to introduce mutations in comparison to the templates, or a polymerase that will discontinue the polynucleotide synthesis prematurely to effect the arrest of the reaction.
Further modifications will be described below.
In a further aspect the invention relates to a method of identifying polypeptides exhibiting improved properties in comparison to naturally occurring polypeptides of the same bioactivity, whereby a library of recombined homologous polynucleotides produced by the above process are cloned into an appropriate vector, said vector is then transformed into a suitable host system, to be expressed into the corresponding polypeptides and displayed, said polypeptides are then screened in a suitable assay, and positive results selected.
In a still further aspect the invention relates to a method for producing a polypeptide of interest as identified in the preceding process, whereby a vector comprising a polynucleotide encoding said polypeptide is transformed into a suitable host, said host is grown to express said polypeptide, and the polypeptide recovered and purified.
Finally, in further final aspects the invention relates to a recombined/shuffled protein, which is obtainable by any of the methods according to the invention, and which is a recombined/shuffled protein comprising the sequences disclosed herein (vide infra).
In those final aspects of the invention, the term xe2x80x9cobtainablexe2x80x9d denotes that said protein is preferable obtained by a method according to the invention. However a prior art known recombination/shuffling technique such as those described in WO 95/22625 or WO 95/17413 may be used too, either alone or in combination with a method according to the invention, in order to obtain said recombined protein.
Accordingly, further final aspect of the invention are;
a recombined/shuffled protease obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type proteases;
a recombined/shuffled lipase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial wild-type sequences
a recombined/shuffled Pseudomonas lipases obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two of the different wild-type Pseudomonas lipases;
a recombined/shuffled xylanase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type xylanases;
a recombined/shuffled cellulase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type cellulases;
a recombined/shuffled amylase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type amylases
a recombined/shuffled laccase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type laccases;
a recombined/shuffled phytase obtainable by any of the methods according to the invention, and comprising a recombined sequence, which at least contain two different partial sequences from at least two different wild-type phytases.
DEFINITIONS
Prior to discussing this invention in further detail, the following terms will first be defined.
xe2x80x9cShufflingxe2x80x9d: The term xe2x80x9cshufflingxe2x80x9d means recombination of nucleotide sequence fragment(s) between two or more homologous polynucleotides resulting in output polynucleotides (i.e. polynucleotides having been subjected to a shuffling cycle) having a number of nucleotide fragments exchanged, in comparison to the input polynucleotides (i.e. starting point homologous polynucleotides).
xe2x80x9cHomology of DNA sequences or polynucleotidesxe2x80x9d In the present context the degree of DNA sequence homology is determined as the degree of identity between two sequences indicating a derivation of the first sequence from the second. The homology may suitably be determined by means of computer programs known in the art, such as GAP provided in the GCG program package (Program Manual for the Wisconsin Package, Version 8, August 1994, Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA 53711)(Needleman, S. B. and Wunsch, C. D., (1970), Journal of Molecular Biology, 48, 443-453).
xe2x80x9cHomologousxe2x80x9d: The term xe2x80x9chomologousxe2x80x9d means that one singlestranded nucleic acid sequence may hybridize to a complementary single-stranded nucleic acid sequence. The degree of hybridization may depend on a number of factors including the amount of identity between the sequences and the hybridization conditions such as temperature and salt concentration as discussed later (vide infra).
Using the computer program GAP (vide supra) with the following settings for DNA sequence comparison: GAP creation penalty of 5.0 and GAP extension penalty of 0.3, it is in the present context believed that two DNA sequences will be able to hybridize (using low stringency hybridization conditions as defined below) if they mutually exhibit a degree of identity preferably of at least 70%, more preferably at least 80%, and even more preferably at least 85%.
xe2x80x9cheterologousxe2x80x9d: If two or more DNA sequences mutually exhibit a degree of identity which is less than above specified, they are in the present context said to be xe2x80x9cheterologousxe2x80x9d.
xe2x80x9cHybridization:xe2x80x9d Suitable experimental conditions for determining if two or more DNA sequences of interest do hybridize or not is herein defined as hybridization at low stringency as described in detail below.
Molecules to which the oligonucleotide probe hybridizes under these conditions are detected using a x-ray film or a phosphoimager.
xe2x80x9cprimerxe2x80x9d: The term xe2x80x9cprimerxe2x80x9d used herein especially in connection with a PCR reaction is an oligonucleotide (especially a xe2x80x9cPCR-primerxe2x80x9d) defined and constructed according to general standard specifications known in the art (xe2x80x9cPCR A practical approachxe2x80x9d IRL Press, (1991)).
xe2x80x9cA primer directed to a sequence:xe2x80x9dThe term xe2x80x9ca primer directed to a sequencexe2x80x9d means that the primer (preferably to be used in a PCR reaction) is constructed to exhibit at least 80% degree of sequence identity to the sequence fragment of interest, more preferably at least 90% degree of sequence identity to the sequence fragment of interest, which said primer consequently is xe2x80x9cdirected toxe2x80x9d. The primer is designed to specifically anneal at the sequence fragment or region it is directed towards at a given temperature. Especially identity at the 3xe2x80x2 end of the primer is essential.
xe2x80x9cFlankingxe2x80x9d The term xe2x80x9cflankingxe2x80x9d used herein in connection with DNA sequences comprised in a PCR-fragment means the outermost partial sequences of the PCR-fragment, both in the 5xe2x80x2 and 3xe2x80x2 ends of the PCR fragment.
xe2x80x9cPolypeptidexe2x80x9d Polymers of amino acids sometimes referred to as proteins. The sequence of amino acids determines the folded conformation that the polypeptide assumes, and this in turn determines biological properties and activity. Some polypeptides consist of a single polypeptide chain (monomeric), whereas other comprise several associated polypeptides (multimeric). All enzymes and antibodies are polypeptides.
xe2x80x9cEnzymexe2x80x9d A protein capable of catalysing chemical reactions. Specific types of enzymes to be mentioned are such as amylases, proteases, carbohydrases, lipases, cellulases, oxidoreductases, esterases, etc. Of specific interest in relation to the present invention are enzymes used in detergents, such as proteases, lipases, cellulases, amylases, etc.
The present invention relates in its first aspect to a method for the construction of a library of recombined homologous polynucleotides from a number of different starting single or double stranded parental DNA templates and primers by induced template shifts during an in vitro polynucleotide synthesis using a polymerase, whereby
A. extended primers or polynucleotides are synthesized by
a) denaturing a parental double stranded DNA template to produce single stranded templates,
b) annealing said primers to the single stranded DNA templates,
c) extending said primers by initiating synthesis by use of said polymerase,
d) cause arrest of the synthesis, and
e) denaturing the double strand to separate the extended primers from the templates,
B. a template shift is induced by
a) isolating the newly synthesized single stranded extended primers from the templates and repeating steps A.b) to A.e) using said extended primers produced in (A) as both primers and templates, or
b) repeating steps A.b) to A.e),
C. the above process is terminated after an appropriate number of cycles of process steps A. and B.a), A. and B.b), or combinations thereof, and
D. optionally the produced polynucleotides are amplified in a standard PCR reaction with specific primers to selectively amplify homologous polynucleotides of interest.
According to the invention the polymerase may be a DNA or a RNA polymerase, specific polymerases to be mentioned are such as DNA polymerases like T4 polymerase, T7 polymerase , E. coli DNA polymerase I or the Klenow fragment of DNA polymerase I, or thermostable polymerases such as Taq, Amplitaq, Vent, Pwo.
One of the advantages of the invention is that it makes it possible to control the length of the extension of the primers in the reaction in a convenient manner.
This can be accomplished by various means such as choice of polymerase, the physical and chemical conditions during the action of the polymerase, e.g. pH, temperature, buffer, salt concentration, and addition of various chemicals.
It is known that various polymerases carry out the DNA synthesis at different rates (nucleotides incorporated pr. second). For example has the Klenow fragment of DNA polymerase I a limited extension rate compared to e.g. the Taq polymerase (Sambrook et al. 1989).
Polymerases also display differences in processivity, which is the average number of nucleotides incorporated before the polymerase dissociates from the template/extended primer; again the Klenow polymerase is an example of a polymerase with limited processivity.
The choice of polymerase is therefore an important means in controlling the average extension of the primers.
These conditions may also exert an influence on the fidelity of the polymerase (the rate by which point mutations are introduced; HIV reverse transcriptase is an example of a polymerase of low fidelity), a parameter useful in combining shuffling and mutagenesis.
In specific embodiments various modifications can be made in the process of the invention. For example it is advantageous to apply a defective polymerase either an error-prone polymerase to introduce mutations in comparison to the templates, or a polymerase that will discontinue the polynucleotide synthesis prematurely to effect the arrest of the reaction. Such a defective polymerases that could be mentioned is a Klenow polymerase having low processivity.
In another embodiment of the invention polymerase will be added after each cycle, if the polymerase used is not thermostable.
According to the invention the starting single or double stranded parental templates may be different in that they contain different point mutations in the same native polynucleotide (gene), or they can be homologous polynucleotides (genes) isolated from nature, which may be amplified by PCR, or they can be combinations thereof. The templates used in the process of the invention are hereby homologous showing an identity at the DNA level of e.g. more than 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, or even more than 50% identity.
It may be advantageous to use pre-selected templates comprising mutations with improved properties of interest. The present recombination method of the invention will then recombine said improved mutations for subsequent screening for even further improvements in the properties of interest.
Said pre-selected templates with improved properties of interest may have been identified by standard procedures in the art comprising e.g. i) error-prone PCR of templates of interest followed by ii) screening/selection for templates with improved characteristic of interest. The mutagenesis frequency (low or high mutagenesis frequency) of the error-prone PCR step is preferably adjusted in relation to the subsequent screening capacity, i.e. if the screening capacity is limited the error-prone PCR frequency is preferably low. (i.e. one to two mutations in each template) (see WO 92/18645 for further details).
The arrest of the polymerase reaction in step A.d) may as indicated above be obtained in different ways, such as by raising the temperature, or adding specific reagents as described in WO 95/17413.
When raising the temperature for this purpose, it is preferred to use temperatures between 90xc2x0 C. and 99xc2x0 C.
When using chemical agents DMSO is a possibility. Appropriate procedures are mentioned in e.g. WO 95/17413.
The process of the invention uses annealing of the primers to the templates in step 1.b. In this context the annealing may be random or specific, meaning either anywhere on the polynucleotide or at a specific position depending on the nature of the primer.
Also, the primers to be used may be completely random primers (NNNNNNNNNNN)(SEQ ID NO: 1) (N meaning a mixture of the four bases (A, T, G, C) is used at a particular position in the primer during synthesis), semi-random primers, or specific primers.
If the extended primers produced are to be separated from the primers during the process it is convenient to use labeled templates in order to provide a simple means for separation, a preferred marker is biotin or digoxigenin.
According to the invention the number of cycles necessary will be less than 500, in most cases less than 200, and normally less than 100 cycles.
In an embodiment of the invention the above in vitro shuffling is combined with a subsequent in vivo shuffling by methods such as those described in WO 97/07205.
In its second aspect the invention relates to a method of identifying polypeptides exhibiting improved properties in comparison to naturally occurring polypeptides of the same bioactivity, whereby a library of recombined homologous polynucleotides produced by the above process are cloned into an appropriate vector, said vector is then transformed into a suitable host system, to be expressed into the corresponding polypeptides, said polypeptides are then screened in a suitable assay, and positive polypeptides selected.
In an embodiment it is contemplated that the polypeptides of interest encoded by the shuffled library are expressed as a suitable fusion protein (e.g. as a hybrid with gIII of bacteriphage M13/fd) in order to display said recombined polypeptide on the surface of phage or bacteria.
In a third aspect the invention relates to a method for producing a polypeptide of interest as identified in the preceding process, whereby a vector comprising a polynucleotide encoding said polypeptide is transformed into a suitable host, said host is grown to express said polypeptide, and the polypeptide recovered and purified.
The use of partial random (semi-random) or completely random primers (mixtures of bases in a selected number or all positions in the primer) as initiation point for the DNA synthesis provide certain novel possibilities for the combined use of shuffling and random mutagenesis.
It is often associated with difficulties to obtain an in vitro recombination of polynucleotides that display relatively limited homology. By the use of an embodiment of the invention even very diverse polynucleotides can be forced to recombine.
According to that embodiment at least two templates (or two pools of diverse templates) are applied. The novel synthesis of the one polynucleotide can then be based on only one strand (i.e. either the sense or the anti-sense strand), and the synthesis of the other polynucleotide is based the opposite strand.
This can be accomplished by isolating the complementary strands from the two templates, e.g. by having these strands labeled by biotin. Synthesis of DNA is initiated by annealing either specific, partly random or completely random primers to these templates and adding a suitable polymerase. This can be performed as either separate reactions for the different templates or in just one reaction. Synthesis should preferably be performed under conditions that favor production of relatively short new fragments. These fragments can subsequently be isolated from the templates based on the affinity label. A PCR reaction is carried out on these fragments and as the starting material originates from two different strands, the newly synthesized fragments must recombine in order to produce full length PCR productsxe2x80x94a kind of forced recombination.
Also for this embodiment rapid PCR with short or no extension time can be applied advantageously in order to enhance recombination, especially if pools of templates are used for the two strands.
The length of the primer and the annealing temperature utilized in the process determines if random primers will anneal and the number of mismatches between the template and the primer that can be accommodated. By varying the primer length and the annealing temperature the method of the invention provides a means for achieving random mutagenesis within a certain nucleotide window representing the length of the primer. The method of the invention thereby provides substantial benefits compared to other random mutagenesis approaches, especially the high probability for several base substitutions close to each other in the primary sequence, e.g. the use of a completely random 20xe2x80x2 mer (mixture of all four nucleotides in all 20 positions) will according to theory under given experimental conditions give a certain reasonably high probability for having several base substitutions close to each other.
Error prone PCR (=high mutagenesis frequency PCR) does not provide this possibility. Error prone PCR provides a very low probability for having more than one base substitution within one codon (coding for one amino acid in a translated polypeptide).
Obviously the substitution of only one base within a codon doesn""t provide total random mutagenesis (at protein level) as only a limited set of amino acid substitutions can be obtained by one base substitution at DNA level (e.g. Methionine encoded by ATG-codon requires three base substitution to become the TGT or TGC-codon encoding Cysteine).
In one embodiment of the invention the process is therefore performed by using random or semi-random primers having a length of from 6 to 200 bp, preferably from 10 to 70 bp, and better from 15 to 40 bp.
One of the advantages in the method of the invention is the robustness. In some embodiments the constant presence of full length template provides a further advantage avoiding PCR contamination problems. Furthermore it is much less laborius, with less hands on, than other described methods, thereby providing excellent possibilities for automation.
PCR-primers:
The PCR primers are constructed according to the standard descriptions in the art. Normally they are 10-75 base-pairs (bp) long. However, for the specific embodiment using random or semi-random primers the length may be substantially longer as indicated above.
PCR-reactions:
If not otherwise mentioned the PCR-reaction performed according to the invention are performed according to standard protocols known in the art.
The term xe2x80x9cIsolation of PCR fragmentxe2x80x9d is intended to cover as broad as simply an aliquot containing the PCR fragment. However preferably the PCR fragment is isolated to an extend which remove surplus of primers, nucleotides templates etc.
In an embodiment of the invention the DNA fragment(s) is(are) prepared under conditions resulting in a low, medium or high-random mutagenesis frequency.
To obtain low mutagenesis frequency the DNA sequence(s) (comprising the DNA fragment(s)) may be prepared by a standard PCR amplification method (U.S. Pat. No. 4,683,202 or Saiki et al., (1988), Science 239, 487-491).
A medium or high mutagenesis frequency may be obtained by performing the PCR amplification under conditions which increase the misincorporation of nucleotides, for instance as described by Deshler, (1992), GATA 9(4), 103-106; Leung et al., (1989), Technique, Vol. 1, No. 1, 11-15.
It is also contemplated according to the invention to combine the PCR amplification (i.e. according to this embodiment also DNA fragment mutation) with a mutagenesis step using a suitable physical or chemical mutagenizing-agent, e.g., one which induces transitions, transversions, inversions, scrambling, deletions, and/or insertions.
Expressing the Recombinant Protein from the Recombinant Shuffled Sequences
Expression of the recombinant protein encoded by the shuffled sequence in accordance with the second and third aspect of the present-invention may be performed by use of standard expression vectors and corresponding expression systems known in the art.
Screening and Selection
In the context of the present invention the term xe2x80x9cpositive polypeptide variantsxe2x80x9d means resulting polypeptide variants possessing functional properties which has been improved in comparison to the polypeptides producible from the corresponding input DNA sequences. Examples, of such improved properties can be as different as e.g. enhance or lowered biological activity, increased wash performance, thermostability, oxidation stability, substrate specificity, antibiotic resistance etc.
Consequently, the screening method to be used for identifying positive variants depend on which property of the polypeptide in question it is desired to change, and in what direction the change is desired.
A number of suitable screening or selection systems to screen or select for a desired biological activity are described in the art. Examples are:
Strauberg et al. (Biotechnology 13: 669-673 (1995) describes a screening system for subtilisin variants having Calcium-independent stability;
Bryan et al. (Proteins 1:326-334 (1986)) describes a screening assay for proteases having a enhanced thermal stability; and
PCT-DK96/00322 describes a screening assay for lipases having improved wash performance in washing detergents.
An embodiment of the invention comprises screening or selection of recombinant protein(s), wherein the desired biological activity is performance in dish-wash or laundry detergents. Examples of suitable dish-wash or laundry detergents are disclosed in PCT-DK96/00322 and WO 95/30011.
If, for instance, the polypeptide in question is an enzyme and the desired improved functional property is the wash performance, the screening may conveniently be performed by use of a filter assay based on the following principle:
The recombination host cell is incubated on a suitable medium and under suitable conditions for the enzyme to be secreted, the medium being provided with a double filter comprising a first protein-binding filter and on top of that a second filter exhibiting a low protein binding capability. The recombination host cell is located on the second filter. Subsequent to the incubation, the first filter comprising the enzyme secreted from the recombination host cell is separated from the second filter comprising said cells. The first filter is subjected to screening for the desired enzymatic activity and the corresponding microbial colonies present on the second filter are identified.
The filter used for binding the enzymatic activity may be any protein binding filter e.g. nylon or nitrocellulose. The topfilter carrying the colonies of the expression organism may be any filter that has no or low affinity for binding proteins e.g. cellulose acetate or Durapore(copyright). The filter may be pre-treated with any of the conditions to be used for screening or may be treated during the detection of enzymatic activity.
The enzymatic activity may be detected by a dye, fluorescence, precipitation, pH indicator, IR-absorbance or any other known technique for detection of enzymatic activity.
The detecting compound may be immobilized by any immobilizing agent e.g. agarose, agar, gelatin, polyacrylamide, starch, filter paper, cloth; or any combination of immobilizing agents.
If the improved functional property of the polypeptide is not sufficiently good after one cycle of shuffling, the polypeptide may be subjected to another cycle.
In an embodiment of the invention wherein homologous polynucleotides representing a number of mutations of the same gene is used as templates at least one shuffling cycle is a back-crossing cycle with the initially used DNA fragment, which may be the wild-type DNA fragment. This eliminates non-essential mutations. Non-essential mutations may also be eliminated by using wild-type DNA fragments as the initially used input DNA material.
Also contemplated to be within the invention is polypeptides having biological activity such as insulin, ACTH, glucagon, somatostatin, somatotropin, thymosin, parathyroid hormone, pituary hormones, somatomedin, erythropoietin, luteinizing hormone, chorionic gonadotropin, hypothalamic releasing factors, antidiuretic hormones, thyroid stimulating hormone, relaxin, interferon, thrombopoeitin (TPO) and prolactin.
A requirement to the starting parental DNA sequences, encoding the polypeptide(s), to be shuffled, is that they are at least 50%, 60%, 70%, 80%, 90%, or 95% homologous. DNA sequences being less homologous will have less inclination to interact and recombine.
It is also contemplated according to the invention to shuffle parental polynucleotides that are homologous as indicated above originating from wild type organisms of different genera.
Further, the starting parental templates to be shuffled may preferably have a length of from about 50 bp to 20 kb, preferably about 100 bp to 10 kb, more preferred about 200 bp to 7 kb, especially about 400 bp to 2 kb.
The starting parental DNA sequences may be any DNA sequences including wild-type DNA sequences, DNA sequences encoding variants or mutants, or modifications thereof, such as extended or elongated DNA sequences, and may also be the outcome of DNA sequences having been subjected to one or more cycles of shuffling (i.e. output DNA sequences) according to the method of the invention or any other method (e.g. any of the methods described in the prior art section).
When using the method of the invention the resulting recombined homologous polynucleotides (i.e. shuffled DNA sequences), have had a number of nucleotide fragments exchanged. This results in replacement of at least one amino acid within the polypeptide variant, if comparing it with the parent polypeptide. It is to be understood that also silent exchanges are contemplated (i.e. nucleotide exchange which does not result in changes in the amino acid sequence).
MATERIALS AND METHODS
Specific Method Used in the Examples:
A) DNA encoding different enzyme variants of the same gene or different enzymes having the same type of activity encoded by homologous genes are mixed. The DNA is provided as either PCR fragments, plasmid, phage or genomic DNA.
B) The resulting pool of DNA is mixed with DNA Polymerase, dNTP, a suitable buffer and primers (being either random oligomers (length of 6-30 nucleotides) or specific oligomers (length of 6-50 nucleotides) or a combination of both types).
C) The PCR mixture is put into a PCR thermocycler (either cold or hot) in a suitable tube.
c1) The thermocycler is heated to a temperature of 90-100xc2x0 C. for a period of time(typically 1-10 min) in order to denature the DNA templates.
c2) Thereafter the following procedure (cycle) is followed (repeated): The template is denatured (typically 90-100xc2x0 C. for 0-5 minutes). Then the temperature is lowered (typically to a value between 10xc2x0 C. and 90xc2x0 C. for 0-5 minutes) to allow annealing of the primer to the single stranded template. Now the temperature is raised again to denaturation temperature (90-100xc2x0 C.) allowing small extension of the primer to be synthesized by the DNA polymerase during ramping. Alternatively a short extension period (typically 0-30 seconds at 70-75xc2x0 C.) can be introduced to allow larger extensions of the primers to be generated. When the temperature reaches a value where denaturation takes place, the extended primers and templates are again separated. This procedure can be repeated (typically between 1 to 99 cycles).
D) Having performed the desired number of cycles the generated small DNA polymers can be purified from the oligomers used as primers. One way is to isolate and clone a specific amplified band containing the gene coding for the polypeptide of interest into a suitable vector. This can be done either on agarose gel (typically isolating fragments between 50 to 1000 base pairs), by beads (using an affinity label on either templates or primers) or through columns.
E) Then the purified (or the not purified) DNA polymers can be assembled in a standard PCR reaction (for instance 94xc2x0 C., 5 minutes, (94xc2x0 C., 30 sec; 55xc2x0 C., 30 sec; 72xc2x0 C., 2 min)*25, 72xc2x0 C. 5 minutes, 4xc2x0 C.).